February 10, 2005For a few days at the end of December, an asteroid named 2004 MN4 looked like it might be Earth's biggest impact threat. Based on available information about its orbit, astronomers gave odds of 1 in 37 that 2004 MN4 would strike Earth April 13, 2029. But astronomers found images of the asteroid taken before its discovery, giving them a longer arc of its orbit, and the collision threat evaporated. It appeared the rock would miss Earth by 40,000 miles (64,400 kilometers).

Now, radar measurements suggest MN4 will miss us by half that distance — and come so close Earth's gravity could rip it apart.

Between January 27 and 30, a team led by Lance Benner of the Jet Propulsion Laboratory in Pasadena, California, tracked the asteroid using the enormous Arecibo radio telescope in Puerto Rico. By bouncing radio waves off the asteroid, the astronomers received precise information about its position and speed that enabled them to plot the object's course over the next 24 years with great accuracy.

The results shocked some astronomers. The new orbit indicates the asteroid will miss Earth by 22,000 miles (35,400 km), passing just inside the belt of geostationary satellites.

A miss is still a miss, so what's the big deal? At first glance, the change in the miss distance doesn't seem surprising. Astronomers are constantly updating comet and asteroid orbits, and changes are expected.

But for 2004 MN4, the change in the miss distance was greater than the error computed in the December analyses. Put another way, 2004 MN4 is now outside the uncertainty box — the region astronomers thought would contain the object's most likely locations on April 13, 2029.

The asteroid, whose chance of striking Earth was once computed to be improbably high, has presented us with the improbable once again. Scientists place great store in their error estimates — sometimes too much. One of the most prominent asteroid researchers, Clark Chapman of the Southwest Research Institute (SWRI) in Boulder, Colorado, notes that who deal with low-probability, high-consequence events, like airplane crashes, place very little faith in error estimates. Perhaps, he suggests, this is a lesson the asteroid community needs to learn.

But there's another reason for concern. According to Dan Durda, another SWRI astronomer, 2004 MN4 is likely to be a "rubble-pile" asteroid, consisting of material only loosely held together by gravity. Because the asteroid will pass us at just 2.5 times Earth's diameter, tidal forces could tear it apart. The result would be a trail of rocks drifting slowly apart with the passage of time. One or more of these might hit Earth in the more distant future, creating a spectacular fireball as it burns up in the atmosphere.

Although a miss in 2029 is virtually certain, if MN4 survives its Earth flyby, astronomers cannot rule out potential collisions in the 2030's. Therefore, 2004 MN4 still holds at 1 on the Torino impact hazard scale, a classification designed to quantify the impact risk of near-Earth asteroids (similar to the Ritcher scale for earthquakes).

Clark Chapman says the past few weeks have been "educational for the asteroid impact community," and he refers to 2004 MN4 as the "most significant event, by far, in decades."

So, take note: On Friday, April 13, 2029, from dark-sky sites throughout Europe, 2004 MN4 will look like a 3rd-magnitude star. It will be moving a quarter of the way across the sky in just an hour, its motion among the stars clearly evident.

And maybe, just maybe, you'll see an asteroid die.

Bill Cookeis an astronomer with NASA's Marshall Space Flight Center in Huntsville, Alabama. His article, "Killer impact," appeared in the December 2004 issue of Astronomy.